Polyimide resins, such as those commercially available as ULTEM® brand resins, have excellent Flame, Smoke and Toxicity (FST) properties and mechanical properties and high temperature capability, but are difficult to bond and retain the FST requirements that are critical as for aircraft, marine and some rail applications. Bonding of sheets of polyimide foam for transportation applications or for radome construction where further machining and the retention of electrical properties such as uniform radar transparency are important is also difficult. Production of interior aircraft panels requires the application of decorative films over large areas, which requires additional time in the mold to develop a high strength bond for current adhesive systems adding time and cost to production, especially if the adhesive has a limited shelf-life. Many of the existing adhesives require high temperature cures, degrade the flame, smoke and toxicity performance or produce a gummy line when additional machining is performed at the bond line.
There are a number of adhesives that can be used for bonding polyimideresin in its various forms, but many of those adhesives have only limited applicability since they fail to meet all but a few of the criteria. Most of the melt adhesives are based on flammable polymers, such as polyvinyl acetate, functionalized polyesters, etc. Such adhesives contribute sufficient fuel and even smoke and toxic combustion products and frequently adhere primarily through mechanical bonding. In the case of polyimide foam, bonding with these adhesives would require sufficient material to fill the open surface cells, providing even more fuel. Polyurethanes are highly effective adhesives but they give off toxic chemicals while burning. Epoxies are similar to the polyurethanes in that they may provide excellent adhesion but epoxy adhesives can be very smoky and require extended cure time thus extending production time. Epoxy adhesives also have limited shelf life and may require refrigerated storage. Phenolic adhesives may provide very low fuel, but generally require high temperature cure and they have limited shelf life.
For critical applications, large blocks of foam had to be chosen from materials that could be made into larger blocks and then machined or bonded. Any inconsistencies in electrical properties, such as inconsistent radar wave absorption, are problems that would have to be compensated for electrically or by use of additional devices.
Therefore, there is a need for an adhesive capable of bonding polyimide resin containing materials to each other where the adhesive has excellent Flame, Smoke and Toxicity (FST) properties, excellent electrical properties, as well as excellent mechanical properties. Such adhesives show great utility when used to bond polyimide materials to each other, and/or bond materials to polyimide resins.